Metal chelates are useful sources for the correction of minor elemental deficiencies in soils used for growing ornamentals, fruits and vegetables. Iron is an essential micro-nutrient. While it is widely distributed in soils and fertilizers, it seldom needs to be applied to crops except in alkaline soils. Many species of plants become chlorotic in such soils unless fertilized with salts soluble under alkaline conditions. An example, is the requirement for such a soluble iron source by pineapples grown on manganiferous soils.
Certain organic chemicals, known as chelating agents, form ring compounds in which a polyvalent metal such as iron is held firmly between two or more bridge atoms. Such bonding, by ring formation, is known as chelation. Among the best chelating agents known are N-(2-hydroxyethyl)ethylenediaminetriacetic (HO-EDTA) acids. They successfully chelate iron, copper, zinc, manganese and calcium. Iron, copper and zinc are agriculturally important but are converted to insoluble hydroxides or insoluble basis salts at levels above about pH 6 or 7 HO-EDTA chelates keep these trace elements soluble and available to the plants in soils up to and over about pH 10 in the case of iron.
Commercially, the preferred iron chelate source has been ferric HO-EDTA.
It is available as a 5% Fe.sup.+3 (equivalent) aqueous solution, or as the crystallized salt. It is also known as HO-EDT.Iron III; or [(N-(2-hydroxyethyl)-ethylenedinitrilo)triaceto] ferrate (CAS Registry No. 17084-02-5). Its formula is accepted as ##STR2##
This compound has been commercially prepared by combining a trisodium hydroxy-EDTA (Merck Index 9622 - CAS Registry No. 139-89-9) with Fe.sub.2 (SO.sub.4).sub.3. This HO-EDTA salt has the accepted formula ##STR3##
This previous procedure generated Na.sub.2 SO.sub.4 as a by-product which was removed by filtration, after a costly brine cooling, as the decahydrate salt. Yield losses during the Na.sub.2 SO.sub.4.1OH.sub.2 O filtration were approximately 10.degree.-20%. Also the product solution was found to precipitate additional Na.sub.2 SO.sub.4 during subsequent storage and/or shipment. The Na.sub.2 SO.sub.4 removed was disposed in land fill as there is little economic demand for such impure "salt-cake".
Ferric HO-EDTA can also be prepared from ferric nitrate. The product is more expensive than that prepared from Fe.sub.2 (SO.sub.4).sub.3 but is more stable concerning sedimentation of sodium sulfate.